Problems are encountered when attempting to pump liquids, and particularly mixtures of liquids and gases, at low absolute suction pressures and low flow rates using a centrifugal device. These problems arise largely from the gas and vapour content of the flow, which is more prominent at lower pressures. These may be dissolved gases which separate at low pressures or the gaseous phase of the liquid itself.
At high flow rates in the order of hundreds of litres per minute, the flow velocity is such that entrained vapour tends to be purged from the system in the high velocity fluid flow. However, known centifugal pumps are unable to operate at low pressures of the order of 23 to 25 inches of mercury and low flow rates of the order of 300 mls per minute. In fact these known pumps are unable to accommodate significant quantities of gas and vapour at these pressures, regardless of the flow rate.
Problems are also encountered in providing reliable long lived seals for these pumps.